The present invention relates to a demodulator and a demodulation scheme which combines both a Maximum Likelihood Sequence Estimator (MLSE) and Symbol Based detection structure and operation for optimum performance for channels with and without delay spread, and more particularly to such a demodulator and demodulation scheme used in conjunction with a Time Division Multiple Access (TDMA) cellular standard. Even more particularly, the present invention relates to such a demodulator and demodulation scheme used in conjunction with an IS-136 North American TDMA Cellular Standard operated at cellular and PCS frequencies.
In mobile cellular communications systems, a signal transmitted from or received by a mobile station and received by or transmitted from a base station is subject to certain types of corruption of the signal. One typical type of corruption is fading. Fading relates from fluctuations in signal strength that are due to the mobility and speeds of the mobile stations. Fading directly relates to the rate of the mobile station's speed. Typically, a mobile station moving at about 60 mph will induce a "fast fading" environment. Mobile stations moving at less than 10 mph, on the other hand, provide a "slow fading" environment. The associated constructive and destructive interference of the signal results from the changing distances between the base station and the mobile station, and the presence of multiple copies of a transmitted signal with different phase-offsets arriving at a receiver simultaneously.
Additionally, there is corruption induced from "delay spreading", or "multipath". Delay spreading, or multipath, occurs when the signal transmitted from the mobile has been deflected from large structures such as buildings and mountains creating multiple distinguishable time-offset copies at the receiver.
A "delay spread signal" also exhibits fading because of the mobility of the mobile station. A "flat-fading signal" by definition does not exhibit delay spread, but does exhibit fading. In general, 90% of all incoming signals from a mobile communication system are flat-fading signals (and thus do not exhibit delay spread) and less than 10% experience multipath interference (and thus exhibit both fading and delay spread).
A conventional TDMA receiver optimizes the receiver for reception of the multipath distorted signal by implementing an equalizer to receive a transmitted burst. The IS-137A and IS-138A (receiver/transmitter performance specification for the IS-136A) standards all use equalizers at a receiver. If the receiver has a non-equalizer option, it must be manually turned on or off at the receiver.
Unfortunately, if the equalizer is used in a "fast flat-fading" channel, degradation of about 2-3 dB usually occurs in a signal. If the fast fading rate is larger than 100 hertz (equivalent to mobile speed of about 100 kilometers per hour at cellular band and 50 kilometers per hour at PCS band respectively for IS-136 channels) then performance loss is about 2-4 dB as compared to a receiver optimized for the fast flat-fading channel. This occurs because the receivers have been designed to operate without any prior knowledge of channel status.
Since such performance loss occurs more than 90% of the time it is desirable to automatically switch the mode of the receiver based upon the channel status.
A conventional cellular receiver uses an MLSE equalizer and employs Diversity Combining and Least Means Square (LMS) channel tracking algorithms.
A conventional MLSE receiver performs demodulation by selecting data sequences with the highest probability of transmission. Many forms of MLSE receivers have successfully implemented corrections to inter-symbol interference (ISI) encountered in mobile radio channels. But while MLSE receivers have a desirable structure to equalize fading channels with delay spreads and multipath characteristics, they are not the best demodulators when applied to flat-fading channels with no delay spread. This is particularly the case if the channel estimator of the receiver produces incorrect channel parameters that determine tap weights for MLSE due to a fast fading rate or a low signal strength in a training sequence.
Demodulators using symbol based detection algorithms, however, severely impair the performance on channels having non-zero delay spread.
The present invention advantageously addresses the above and other needs by combining the desirable features of two demodulation modes.